1. Field of the Invention
The present invention relates to synthetic quartz glass having a high transmittance to short-wavelength ultraviolet radiation, such as excimer laser light, and particularly in the vacuum ultraviolet region. The invention also relates to a process for producing such synthetic quartz glass.
2. Prior Art
Synthetic quartz glass, because of its low thermal expansion and its high purity and quality, is used in semiconductor manufacturing applications such as process tubes for heat treatment furnaces employed in silicon wafer oxidation and diffusion operations. In addition, it has a high transmittance to ultraviolet light, which makes it an indispensable material in lithography tools used to fabricate large scale integration (LSI) chips. The specific role of synthetic quartz glass in lithography tools includes its use as a stepper lens material and a reticle or photomask substrate material in the processes of circuit pattern exposure and transfer onto silicon wafers.
As LSI chips continue to become more versatile and higher performing, research and development is actively underway to increase the level of device integration on wafers. Achieving higher device integration requires a high optical resolution capable of transferring very fine patterns. The way to achieve a higher resolution is to shorten the wavelength of the light source. The wavelength of ultraviolet light from the light source primarily used today is 248 nm (KrF excimer laser), although efforts are being made to move to a wavelength of 193 nm (ArF) as quickly as possible. Ultraviolet light having a wavelength of 157 nm (F2) also shows considerable promise, and may well see widespread use in the not-too-distant future.
Quartz glass is generally capable of transmitting ultraviolet light, but transmittance declines in the vacuum ultraviolet region below 200 nm and ceases altogether at wavelengths near 140 nm, which is an absorption region owing to the inherent structure of quartz glass. In the wavelength range above the inherent absorption region, there do exist absorption bands which arise on account of defect structures within the glass. Hence, there can be considerable differences in transmittance depending on the type and extent of defect structures. When quartz glass has a low transmittance at the wavelength used in a lithography tool, the absorbed ultraviolet light is converted to heat energy within the quartz glass. Irradiation thus causes compaction to occur at the interior of the glass, which makes the refractive index of the glass non-uniform. One consequence is that, if the defect structures within the quartz glass are strongly absorbing near the irradiation wavelength, this can lower the transmittance of the glass and also reduce its durability as a material used in lithography tools.
Typical defect structures in quartz glass include Sixe2x80x94Si bonds and Sixe2x80x94Oxe2x80x94Oxe2x80x94Si linkages. Sixe2x80x94Si bonds are sometimes called xe2x80x9coxygen deficiency defects,xe2x80x9d and have absorption bands at 163 nm and 245 nm. Because such oxygen deficiency defects are also precursors of Si. defect structures (Exe2x80x2 centers) which have an absorption band at 215 nm, they cause serious problems not only when F2 (157 nm) is used as the light source, but also with the use of KrF (248 nm) or ArF (193 nm).
Sixe2x80x94Oxe2x80x94Oxe2x80x94Si linkages are known as xe2x80x9coxygen surplus defects,xe2x80x9d and have an absorption band at 325 nm. In addition, Sixe2x80x94OH bonds and Sixe2x80x94Cl bonds exhibit absorption bands near 160 nm. It is therefore important in producing quartz glass having a high transmittance in the vacuum ultraviolet region to suppress the formation of oxygen deficiency defects, and also to hold the hydroxyl groups and chloride groups to low concentrations.
In light of the above, the production of quartz glass for use in the vacuum ultraviolet region is preferably carried out via the soot technique, which is able to minimize the formation of Sixe2x80x94OH bonds; that is, by a process in which silica soot is generated and deposited to form a porous silica matrix, which is then fused and vitrified. However, in synthetic quartz glass production using only the soot technique, many Sixe2x80x94Si bonds form at a hydroxyl group concentration lower than a few tens of parts per million, resulting in a very large absorption near 163 nm. For this reason, prior-art synthetic quartz glass production processes have included post-treatment steps such as hydrogen annealing of the Sixe2x80x94Si bonds that have formed to convert them to less deleterious structures such as Sixe2x80x94H bonds.
It is therefore an object of the invention to provide a process for producing synthetic quartz glass, which process allows the formation of Sixe2x80x94Si bonds to be suppressed, reduces or eliminates the need to repair defect structures by post-treatment following synthetic quartz glass production, and is capable of producing synthetic quartz glass having a high transmittance in the vacuum ultraviolet light region. Another object of the invention is to provide synthetic quartz glass produced by such a process.
We have found that if a stoichiometric excess of oxygen is established in the gas balance during creation of the porous silica matrix in the synthetic quartz glass production process, the formation of Sixe2x80x94Si bonds in the quartz glass can be suppressed. It is then possible to produce a synthetic quartz glass having a high transmittance in the vacuum ultraviolet region.
Accordingly, the invention provides a process for producing synthetic quartz glass, comprising the steps of feeding to a reaction zone a silica-forming raw material gas and a fluorine compound gas from a first nozzle at the center of a burner having a plurality of concentric nozzles, oxygen gas from a second nozzle disposed concentrically outside the center nozzle, and oxygen gas or hydrogen gas or both from a third nozzle disposed concentrically outside the second nozzle; flame hydrolyzing the silica-forming raw material gas in the reaction zone to form fine particles of silica; depositing the silica fine particles on a rotatable substrate in the reaction zone so as to create a porous silica matrix; and fusing the silica matrix.
In a first embodiment of the invention, the oxygen gas fed from the second nozzle is set at a flow rate with respect to the raw material gas flow rate which represents a 1.1- to 3.5-fold stoichiometric excess of oxygen.
In a second embodiment of the invention, the oxygen gas is fed from the burner at an overall flow rate with respect to the sum of the raw material gas flow rate and the overall hydrogen gas flow rate which represents a 1.1- to 3.5-fold stoichiometric excess of oxygen. In the second embodiment, the oxygen gas fed from the second nozzle is preferably set at a flow rate with respect to the raw material gas flow rate which represents a 1.1- to 3.5-fold stoichiometric excess of oxygen.
The porous silica matrix in the foregoing processes of the invention preferably has a density of 0.1 to 1.0 g/cm3.
The invention further provides a synthetic quartz glass produced by fusing and vitrifying the porous silica matrix according to either of the foregoing processes of the invention. The synthetic quartz glass has a hydroxyl group concentration of at most 20 ppm and a fluorine atom concentration of at least 100 ppm.